Outside the Pipe

Understanding Clamp-On Transit-Time Flow Measurement

For many industrial processes, the integrity of fluid containment may be quality or safety critical. Indeed, both factors may often apply. In other words, the boundary between the process fluid and its external environment — i.e., the system of pipes and vessels that contain the fluid — must remain intact at all costs. Along this line, modern, noninvasive, transit-time ultrasonic flow measurement technology offers industrial end-users unique benefits for such quality- and safety-critical scenarios.

Across industries like life sciences, food & beverage and semiconductor, the concern is typically one of process fluid containment for reasons of quality — i.e., not allowing the outside environment to enter the process and thereby contaminate it. Achieving the necessary levels of hygiene and purity in such processes can be both highly expensive and time consuming.

By contrast, industries like chemicals, oil & gas and nuclear power are more typically concerned with process fluid containment for reasons of safety. Here the focus is on preventing the process fluid from escaping to the outside environment, because the fluid may be highly corrosive, toxic or otherwise dangerous. With either of these application requirements, a noninvasive or noncontact flowmeter would be necessary where flow measurement and/or flow monitoring was needed.

In 2005, Borden Chemicals UK Ltd of Cowie, Stirlingshire, in Scotland (now Hexion Specialty Chemicals, www.hexion.com) took decisive action to solve a long-standing problem arising from unreliable, in-line flow measurement devices. The company’s products include high-performance resins and adhesives for the construction, automotive and aerospace industries. In the application under consideration here, formaldehyde was involved.

Reliable flow monitoring was essential in this application to protect an absorption tower from fouling caused by accumulation of the solid form of formaldehyde, known as para-formaldehyde or paraform. Paraform can accumulate due to a reduction in circulation flow caused by deposition or mechanical wear of the circulation pump. Unfortunately, contamination of the in-line flowmeter from liquid-born solids necessitated its regular removal from the process pipe for cleaning. When this cleaning took place, maintenance personnel were at increased risk of exposure to the hazardous chemical. The manufacturing process also had to be shut down during this work, which often lasted for several days before restart, resulting in significant production losses and costs for hazardous waste disposal.

Clamp-on transit-time flowmeters were selected as an alternative solution for this application. Installation was achieved without process disruption, and there was no risk of fluid contact as a result of the flow measurement process. But the real value of the technology was realized during the years that followed. According to the site’s engineering supervisor, “The installation has provided a reliable form of flow measurement in a critical circulation loop, cut costs and reduced the risk of exposure to formaldehyde. And the equipment has been maintenance-free since installation.” Upon follow-up in 2009, the clamp-on transit-time flowmeters remained maintenance-free.

Key Application Benefits The case study provided above illustrates an example of how clamp-on transit-time technology represents a good fit for flow measurement applications where a challenging process liquid is the issue. However, the process liquid often demands a special pipe format or pipe material that is compatible with the liquid properties. Sometimes the pipe schedule or wall thickness is required to meet extreme pressure limits, and often the material itself must be capable of standing up to the corrosion factor of the system. A clamp-on system can take such considerations out of the equation since it does not require penetration of the pipe and is not directly exposed to the process media.

Clamp-on transit-time installation in two-traverse configuration shown both diagramatically and in real-life situation.

Imagine the difficulties and cost of manufacture faced by the oil & gas sector for an inline-type flowmeter, where the specified pipe material might be duplex stainless steel or where the material must be more than perhaps 35 mm thick with a pressure rating of over 300 bar. In another industry, perhaps the required pipe material is a composite, such as fiber-reinforced plastic (FRP/GRP) or PVDF. Is an inline meter available in such a material? Certainly some special inline meters can be found in non-standard materials, often focused on a specific industry and application. But trying to procure a special product is often a cumbersome and time-consuming process that doesn’t always produce satisfactory results.

By contrast, clamp-on transit-time technology provides a viable solution for many complex applications because it can often be retrofitted to an existing process pipe, requiring only the knowledge of pipe specification — i.e., diameter, wall thickness and material. The technology is also capable of handling pipes with internal liners, as well as those with bonded coatings on the interior and exterior. Further, some transit-time clamp-on meters can utilize additional ultrasonic sensors to ascertain both pipe wall thickness and the velocity of sound of the process liquid itself, if this information is not readily available from tables.

Figure 1. Shows the close trending of a clamp-on meter with an inline custody-transfer heat meter. A correlation coefficient of 0.9965 was obtained between the two devices.

Understanding the Technology It is important to understand that two very different forms of ultrasonic clamp-on flow measurement technology can commonly be found in the marketplace today. And while both do indeed employ ultrasound, they are in fact completely different. While this article focuses on applications involving transit-time ultrasonic clamp-on technology for liquids, it is equally important to understand the underlying principles of the other form of ultrasonic clamp-on flow measurement — i.e., Doppler.

While transit-time operates in the time-measurement domain for liquids that range from ultra-pure to those containing low levels of gas bubbles or entrained solids, Doppler operates in the frequency domain and requires the presence of bubbles and/or solids to reflect the ultrasonic pulses propagated into the liquid. A Doppler meter operates by measuring the shift in frequency of received signals relative to the transmitted frequency, where this so-called “Doppler Shift” in frequency is proportional to the velocity of the source of reflection — be it bubbles or solid material — suspended in the flowing liquid.

A transit-time meter employs a functional pair of ultrasonic transducers, which are positioned such that the path along which they transmit and receive ultrasonic pulses is at an angle to the pipe axis. Because of this angle, any fluid flow in the pipe will create a differential in the measured transit times of the alternately propagated ultrasonic pulses.

Those pulses transmitted in the upstream direction will be slowed down as a function of the oncoming fluid flow velocity, while those traveling downstream will be accelerated by an equal amount. The greater the flow velocity, the greater the transit-time difference.

Snell’s Law describes the relationship between adjacent materials and fluid sound velocities during the passage of sound energy and defines resulting angles of refraction at each material interface — i.e., transducer/pipe, pipe/liner or liner/liquid. As stated above, the measured variable in this form of flowmeter is time, and while in essence such a meter is a form of clock, it must maintain incredible precision in the order of <50 picoseconds, or 50 x 10-12 seconds. In this area of time measurement precision, and in the area of digital signal processing, significant advances have been made in the last decade that have greatly advanced the performance, reliability and scope of applications for transit-time ultrasonic flowmeters. So much, in fact, that the inline, multiple-path form of transit-time ultrasonic flowmeters has gained industry approval for custody-transfer and fiscal applications in the oil & gas sector.

Realistic Expectations of Performance Clamp-on transit-time flowmeters are capable of achieving <0.5 percent accuracy under reference conditions, but when installed in the field, it is assumed that bias to the absolute flow value may result from errors in programmed pipe data, such as, for example, wall thickness. As such, allowing for potential bias arising from field installation, the practical expectation for clamp-on accuracy is opened out to a range of +/-0.5 percent to +/-2.0 percent of reading. This expectation is supported by independent evaluations over the past 15 years. To achieve this performance, a minimum of 15 diameters straight inlet run is needed. However, clamp-on transit-time meters are capable of maintaining repeatability of 0.3 percent of reading, regardless of inlet run or any additional field installation errors that may or may not accrue.

Regarding accuracy, clamp-on transit-time flowmeters have shown better than 1 percent of reading in many in-field installations compared to in-line reference meters, drop tests or other methods. Such accuracies require due care and attention to obtaining and programming correct pipe data, as well as installation of the clamp-on transducers in a recommended location. The installation shown in Figure 1, for example, features a 20-inch carbon steel district heating line containing hot water at 90 C to 130 C. The trend graph shows how the clamp-on meter correlates with the existing in-line custody-transfer energy meter.

Continuous research and product development work over the last few years has yielded a significant advance in ultrasonic signal transmission. One important aspect of performance is always reliability, and for a transit-time meter, reliability depends upon signal quality. Many things can be done with modern digital signal processing (DSP) techniques, but one might say that this is treating the symptoms and not the cause. A reliable received signal should reach its maximum amplitude in the fewest number of cycles possible, where the signal is at its clearest and strongest, and where adjacent peak heights for the received signal waveform will be significantly different, thus ensuring robust and accurate time measurement.

Research showed that each pipe material of a certain thickness supported a specific frequency where acoustic attenuation was at its minimum, and thus optimum signal transmission and reception would occur. Based on this research, a transducer has been developed to operate not at one nominal frequency, e.g., 2.0 Megahertz (MHz), but over a band of frequencies approximately 0.5 MHz above and below the nominal frequency. During startup, the transit-time meter scans the frequency band until the optimum frequency is found for the pipe involved, and then operates thereafter at that frequency. One further benefit of this technology is that transducers can be supplied at a higher-than-normal nominal frequency for larger pipe sizes, which again assists with time measurement precision during operation.

Mechanical stability is also important in maintaining robust acoustic transmission, and where a process pipe is prone to expansion and contraction due to temperature change, the clamping mechanism held in place normally by metal bands or straps can become slightly loose, and acoustic transmission might then become impaired or lost, as the flat face of the transducer is no longer held firm to the pipe surface. However, a modern spring-loaded transducer design ensures good contact is maintained at all times. All ultrasonic transducers require some form of flexible coupling material between the emitting face and the pipe surface. This normally takes the form of a gel or grease, but more durable materials, such as adhesives and compressible pads, are now proven in use for periods of several years, thus totally precluding routine maintenance.

This technology is intended only for full pipes containing single-phase fluids. This effectively means liquids with a content of suspended gas or air bubbles limited to 3 percent by volume, or suspended solids of 5 percent by volume. If a pipe liner material has become detached from the interior of the parent pipe, an alternative installation location may be necessary. Where old pipes have become severely internally corroded, or perhaps significantly coated by some attenuating substance such as pipe scale or other foreign material, it may be found that acoustic transmission is difficult, e.g., a single-traverse installation is needed on a pipe where diameter normally prescribes two traverse (single bounce), or operation may simply be impossible. Uniquely, the feasibility of a transit-time flowmeter installation can be proven with no process disruption and minimal cost.

Measuring the Benefits Clamp-on transit-time ultrasonic flowmeters enable end-users to retro-fit an existing process, and thus preclude the need for a process shutdown for installation. As such, clamp-on systems should always merit consideration when selecting a flow measurement technology in today’s industrial and economic environment. Clamp-on flow measurement systems are also advantageous in regard to total installation costs, including health & safety planning with necessary work permits; line emptying and purging; cutting and welding; and the hourly cost of lost production throughput. The potential cost of lost production throughput is naturally dependent upon many factors, but as an illustration, a product flowing in a four-inch line at a typical liquid flow velocity of seven feet per second is a flowrate of 62,832 US gallons per hour, and with a product worth just $0.25 per gallon, this represents a loss of $15,700 per hour, or $125,000 per eight-hour shift. Furthermore, the capital cost of a clamp-on meter essentially stays the same regardless of pipe size, and thus the investment value of this technology increases as a function of line size, with clamp-on meters having been successfully applied to pipes ranging in size from ½” to over 13 feet in diameter.

Consider also the situation where a critical in-line flowmeter has become unreliable or has completely failed. A clamp-on meter can typically be installed immediately adjacent (upstream) of the in-line device with no process disruption and normally within an hour. The clamp-on meter could also be easily removed when the in-line meter is replaced. Clamp-on meters also exist in portable form, allowing temporary, short-term measurements to fulfill such requirements as in-line meter verification, pump efficiency monitoring, process startups and surveys.

Steve Milford is the global business driver for Ultrasonic Flow at Endress+Hauser Flowtec AG, located in Reinach, Switzerland. He has been involved in flow measurement and process control instrumentation for over 26 years, with a focus on ultrasonic technology. During this time he has engineered, installed and commissioned over 500 ultrasonic flowmeters. He has actively been involved with ISO and BSI committees for ultrasonic flow, Mr. Milford can be reached at steve.milford@flowtec.endress.com.